1. Field of the Invention
The present invention relates to grounding switches for gas-insulated high voltage switchgear, and more particularly to metal-enclosed high voltage switchgear utilizing a grounding switch.
2. Prior Art
In gas-insulated high voltage switchgears, grounding switches of two types are used, namely, slow operating switches and fast grounding switches with closing capacity. The invention relates to a grounding switch of the latter design, in which a movable contact is positioned in a mechanism on the grounded enclosure and is driven, for example, by a spring-operated closing device to provide contact with a counter contact at high potential. Such a fast grounding switch should be capable of grounding the high voltage parts also when voltage is applied to them, due to some error, during a closing operation. In the event of such a switching operation, the contacts are exposed to full short-circuit current. To be able to cope with the closing operation without causing contact welding or mechanical damage, the movable contact operates at a high speed and the contacts have an appropriate shape.
FIG. 1 schematically shows a previously known embodiment of a grounding switch of the kind mentioned. High voltage conductor 2 (the main current path) is supported by insulator 3 in grounded metal enclosure 1. Fixed contact 4 (the counter contact) of the grounding switch is arranged on conductor 2, and consists of a sleeve contact with surrounding metal screen 5. Movable contact 6 is in permanent connection with grounded enclosure 1 and is operated by operating device 7. The closing operation is performed by a known spring mechanism and is very quick, whereas the opening operation is performed by means of a motor, or manually, and is relatively slow.
In a closing operation, when movable contact 6 approaches counter contact 4, a flash-over occurs (FIG. 2) and burn damage A occurs in the zone around counter contact 4 (FIG. 3). Such burn damage may considerably reduce the insulating resistance of the switchgear. For voltages up to 300 to 400 kV the extent of the flash-over is about 30 to 50 mm, and the parts around the counter contact can then be designed so that no harmful damage to the voltage stability of the switchgear occurs. At higher voltages (500-800 kV) the flash-over distance is considerably greater and the zone on which damage may occur due to a random flash-over increases rapidly.